This invention relates to an electric discharge machine, and more particularly to an electric discharge machine formed of an electrode and a workpiece installed opposite each other with an insulating working liquid filling the gap therebetween, and used to machine the workpiece by causing an electric discharge across the gap.
FIG. 1 is a schematic structural view of a conventional electric discharge machine. In FIG. 1, an electrode 10 is installed opposite to a workpiece 14 in processing tank 12 through an insulating working liquid 16. A power supply 18 for machining purposes is connected across the electrode 10 and the workpiece 14. The power supply 18 comprises a d.c. power supply 18a, a switching element 18b for interrupting the machining current, a current limiting resistor 18c, and an oscillator 18d for controlling the interrupting operation of the switching element 18b, and is used to supply current intermittently to the gap 20 between the electrode 10 and the workpiece 14.
The aforementioned current I is expressed by I=(E-Vg)/R (where E=voltage of the d.c. power supply 18a, R=resistance of the current limiting resistor 18c, and Vg=interpole voltage). The interpole voltage Vg ranges from 20 to 30 V during the arc discharge and becomes 0 V during short-circuiting, E V in the absence of electric discharge, and 0 V when the switching element 18b is in an OFF state.
Consequently, if the interpole voltage Vg is detected and averaged in a smoothing circuit 22, this value may be used to control the interpole gap; that is, the mean voltage Vs is high when electric discharge is not readily caused when the interpole gap 20 is wide. When the interpole gap 20 is narrow, the mean voltage Vs is lowered because of short-circuiting or readily caused electric discharge. Accordingly, it is possible to control the feeding position of the electrode 10 so as to make the interpole gap 20 roughly constant by means of an oil hydraulic servo mechanism comprising an oil hydraulic pump 28 and an oil hydraulic cylinder 30, if the difference between the mean voltage Vs and a reference voltage Vr is amplified by an amplifier 24 and inputted to an oil hydraulic servo-coil 26.
The most common method of distinguishing between good and bad machining conditions in the conventional electric discharge machine is by observing the mean voltage Vs of the interpole voltage Vg. In other words, when the mean voltage Vs is low, the interpole impedance is also low; this causes short-circuiting and continuous arc discharge. The occurrence of short-circuiting and continuous arc discharge is due to the presence of chips as well as sludge in the interpole gap 20. However, the most dangerous abnormal arc discharge during electric discharge machining is such that once the short-circuiting or continuous arc discharge occurs, carbon is generated by thermal decomposition of the working liquid, and as a result, the electric discharge occurs across the carbon and the workpiece, whereby interpole impedance is increased. That is, despite the fact that the interpole gap is actually narrow, the gap may be judged as wide and normal machining may not be carried out. For this reason, there is a disadvantage in that it is impossible to detect a deteriorated condition in the interpole gap because of an abnormal arc discharge by only observing the mean voltage Vs.